2 Chemical Reaction Notes We have already discussed the fact that sometimes individual atoms (and in the case of polyatomic ions small groups of atoms) can join together to form compounds in order to increase stability and decrease the total amount of chemical potential energy
3 Chemical Reaction Notes Sometimes compounds can come together and they can become more stable still by rearranging atoms.These changes are called chemical reactions, and are different from the nuclear reactions we studied earlier, which are when nuclei come together (or split apart) and create new atoms
4 Chemical Reaction Notes Chemists represent chemical reactions with “equations”
5 Chemical Reaction Notes Equations consist of a list of formulas for all of the reactants (compounds or elements added together to begin a chemical reaction) on the left side and a list of all of the products (compounds or elements which form during a chemical reaction) on the right side
6 Chemical Reaction Notes Reactants and products are separated by an arrowReactants are separated from one another by plus signs (as are products)
7 Chemical Reaction Notes Each formula may be followed by a subscript (solid, liquid, gas, aqueous) to show what state each compound is in.We may represent energy (heat, light, energy) as either a product or a reactant by writing it on the appropriate side of the equation.
8 Chemical Reaction Notes Law of Conservation of Mass: In a chemical reaction the arrangement of atoms will change, but the total number of each type of atom, and therefore the total mass, will remain constant
9 Chemical Reaction Notes All chemical reactions must obey this law and therefore all chemical equations must obey it also.
10 Chemical Reaction Notes In order to ensure that our chemical equations obey the law of cons. of mass, we balance all of our equations.To do this:1. You must make sure that all of the individual compound’s formulas are correct.(Hydrogen, oxygen, nitrogen, and the halogens are all diatomic elements, meaning that whenever they are not found in a compound they form a molecule with themselves and have a formula with a subscript 2)
11 Chemical Reaction Notes 2. Pick any element and place a number (a coefficient) in front of the compound formula to balance out the amount on both sides of the equation.3. You should then balance one element at a time from one side of the equation to the other by placing coefficients in front of the remaining formulas
12 Chemical Reaction Notes 4. Multiply coefficients to get rid of fractions if necessary5. Reduce coefficients if necessary
13 Chemical Reaction Notes Special hints:1. Never change your subscripts2. Balance elements that only appear in one formula on each side first3. After your first “free choice” of a coefficient, you must only use coefficients that balance the elements4. For polyatomic ions, balance them as though they are single “groups”